Recently, there has come to be practiced an optical information recording and reproducing apparatus wherein information can be recorded by radiating a condensed light beam instead of using a magnetic head. The information recorded in a recording medium can be reproduced by receiving the returning light of the light beam condensed and radiated onto the recording medium.
In an apparatus utilizing a photomagnetic phenomenon in the above mentioned optical information recording and reproducing apparatus, the same as in the magnetic system recording and reproducing apparatus (magnetic disk apparatus), the recorded information can be erased and information can be written in again. Therefore, such an apparatus is expected to be extensively used hereafter as an external storage apparatus of a computer.
Now, the conventional photomagnetic recording and reproducing apparatus is of a formation as is shown, for example, in FIG. 1.
A photomagnetic disc 1 is rotated and driven by a spindle motor 2, a light pickup 3 is arranged as opposed to one surface of this disc 1 and a field magnetic coil 4 is arranged as opposed also to the other surface of this disc 1 so that a magnetic field may be applied to the photomagnetic disc 1 part on which the light is condensed and radiated by the light pickup 3 to be able to not only reproduce when there is no need for the magnetic field) but also to record and to erase.
The above mentioned light pickup 3 is controlled for reproducing, recording and erasing by a controller 5.
For example, in the reproducing mode, the controller 5 feeds a comparatively weak constant current corresponding to the reproducing (reading) mode to a light emitting source such as a semiconductor laser (not illustrated) of the light pickup 3 through a constant current circuit 6. The information data read out by the laser light under this constant current is transmitted to the controller 5.
On the other hand, in the recording mode and erasing mode, it is necessary to apply a magnetic field. Therefore, the controller 5 feeds a magnetic field generating current to the field magnetic coil 4 through the constant current circuit 7.
In the above mentioned recording mode and erasing mode, it is necessary to reverse the direction of the magnetic field. Therefore, the controller 5 controls the currents directed reversely to each other to flow through a field magnetic coil 4 in the recording mode and erasing mode.
In the above mentioned photomagnetic recording and reproducing apparatus, in order to prevent the size of magnetic field, applied to the disc 1 by the field magnetic coil 4 based on the, vertical movement of the rotated and driven disc 1, from fluctuating so that recording or erasing can not be positively made, a vertically movable auxiliary magnetic field generating apparatus is provided according to the gazette of a Japanese patent application laid open No. 48803/1984. In this prior art example, the difference between the reference voltage and the level of the detected magnetic field is detected and the auxiliary magnetic field is controlled with the differential output to be held on a predetermined magnetic field level.
Further, in the gazette of a Japanese patent application laid open No. 203258/1984, reflected light of equal light amount will enter respectively the two light receiving surfaces of a two-divided light detector 22 as an information detecting means in case the disc is in a predetermined position and the light amount entering the respective light receiving surfaces will vary in case the disc moves vertically so that the magnetic field level may be controlled from this variation amount and the magnetic field intensity in the recording medium may be kept constant.
The above mentioned prior art example will have no problem when the polarity of the magnetic field detected by the magnetic field detecting means is correct but can not detect even an abnormal state, due to a break of the field magnetic coil or the rare shorting of the insulating member between coils by a defective insulation, when no normal magnetic field is obtained.
Also, when the direction of the current flowing through the magnetic field coil is not normal at the time of a recording or erasing mode, the difference from the reference voltage will be the value of an actual addition and the bias magnetic field can not be normally applied. Therefore, recording or erasing can not be positively made.
Further, the above mentioned prior art example is to keep the magnetic field intensity in the recording medium constant and does not concretely describe a means whereby an abnormality of the magnetism can be detected and a means of prohibiting the recording and erasing operation in case an abnormality of the magnetism is detected.